A base station installed in a radio access network is generally formed from a baseband unit (BBU) for executing baseband processing and the like and an radio frequency (RF) unit (RFU) for executing RF processing and the like. An arrangement in which a BBU is integrated with an RFU is known as the arrangement of such a base station.
To the contrary, in recent years, in a radio access network such as LTE (Long Term Evolution), a base station (base station system) in which it is possible to install a BBU and an RFU at different locations and which is capable of connecting a plurality of RFUs to one BBU via an optical interface is becoming widespread. In such a base station, for example, CPRI (Common Public Radio Interface) has been stipulated as the standard of an interface for connecting an RFU and a BBU. For example, the BBU and RFU transmit digitized baseband signals to each other via an optical interface complying with CPRI. Note that the RFU may also be referred to as a remote radio head (RRH).
In a CPRI line for connecting a BBU and an RFU, due to an enlargement of the bandwidth of a base station, an implementation of multiband, an increase in multiplex number of MIMO, and the like, a large amount of traffic occurs and a necessary transmission capacity can suddenly increase. A large amount of traffic can be accommodated by laying down a number of optical fibers between the BBU and the RFU but the construction of the optical fibers requires a high cost. Although it can be considered to multiplex a number of CPRI lines by wavelength division multiplexing (WDM), this increases the apparatus cost for an optical transceiver and the like.
In view of the above, a technique of compressing a baseband signal and transmitting the compressed signal between the BBU and the RFU has been considered to accommodate a large amount of traffic by a CPRI line while suppressing an increase in cost. PTL 1 proposes some methods for compressing digitized baseband signal samples to be transmitted between a BBU and an RFU. More specifically, compression by Huffman coding, compression by calculation of a primary or higher-order difference between baseband signal samples and coding of the difference, compression based on at least one of a sampling rate, sample width, bandwidth, and modulation type, and the like are proposed. For example, in compression by Huffman coding, the frequencies of occurrence of sampled values are obtained in advance using a series of sampled values of a baseband signal, and sample compression is performed by assigning a shorter code to a sampled value as its frequency of occurrence is higher.